Lathe spindle bearing nuts loosening.

[Ksracer]

Hello guys, hoping somebody might have a few ideas.

I have a 1993 model Kingston HJ-1700 and I'm having problems keeping the large spanner nuts that set preload on the spindle bearings tight. There are two opposing tapered roller bearings (circled in red) on the chuck end with an adjustment nut (circled in blue) behind them and a third support bearing on the outboard end. The nuts have a grub screw that pushes a brass plug against the threads to lock them in place after the adjustment is made.

Twice now I have removed the cover and found both nuts almost a full turn loose, with a gap between them and the bearing. The last time it happened I painted witness marks on them and verified that they are indeed turning in relation to the shaft. If I back the nuts off and tighten the grub screws, it takes quite a bit of force on a 12" long spanner to turn them, but it can be done. Shouldn't really take that much to keep them in place though.

The previous owner mentioned that they had made multiple adjustments over the course of the machines life and I replaced the bearings after I got it thinking maybe they had just worn. The bearings don't seem to be moving on the spindle, the nuts are just rotating away from them.

If anybody has ever experienced this or has any ideas to look at, I would appreciate the insight.

Thanks

 

[technocrat]

Likely your problem is that the shaft is moving when you tighten the nuts, so you can't get preload and tight nuts at the same time. I put a piece of 2x1 clamped in the chuck jaws and resting on the bed to hold the spindle. Then I use a c-spanner to hold the inboard nut and a punch and hammer to set the jam nut tight.

 

[Ksracer]

Likely your problem is that the shaft is moving when you tighten the nuts, so you can't get preload and tight nuts at the same time. I put a piece of 2x1 clamped in the chuck jaws and resting on the bed to hold the spindle. Then I use a c-spanner to hold the inboard nut and a punch and hammer to set the jam nut tight.

I used a piece of wood to jamb the chuck also until I get the preload I want.

For clarity, there is only 1 nut with a grub screw in direct contact with the bearing.

The machine has straight cut gears and I thought maybe they were creating a harmonic that was loosening the nuts, but this model has been in production for over 30yrs. Certainly it didn't survive that long if that was the problem. I'll call Kingston after they re-open, I'm just spitballing here.

 

[ratbldr427]

There are two types of set screws that are used to lock nuts as you describe. The first is just a soft flat tip brass or plastic so as not to damage the threads.
The second and best is a threaded tip insert screw. I have made them by threading the nut with the set screw in place. The only other variation is to use a lead insert(which I will never use again) that holds to good.
The difference in holding power between threaded and flat tips is way more and doesn't damage the threads. Down side is harder to make unless you can find a spare bearing nut and make your own or just thread some scrap with the tips in place.
Double nuts and a French lock between them works but requires a key way.
Another option is Loctite sleeve retainer, good luck on ever trying to move them again.

 

[Bill D]

Any chance power brakes are causing a problem?
Bill D

 

[Ksracer]

Any chance power brakes are causing a problem?
Bill D

Are you referring to an automatic brake?

No, it's set up on a VFD for soft starts (also functions as a phase converter) and I slow it to a stop it with the foot brake.

Speaking of the VFD. The 7.5HP 1800 rpm motor drives the headstock input shaft through a set of 3 BX sized belts that are several feet long. I thought maybe a faulty VFD could cause a motor to run at an unsteady or jerky speed, but I'm having a hard time believing that those belts would transfer enough vibration to cause a problem.

 

[James H Clark]

The correct size AN-series bearing lock nuts need to be used. That is to say that two of the AN-series lock nuts with the correct size locking washer used with the correct tabs locking the two nuts in place. Grub screws are not the right way to do it. i have never seen a lathe worth its salt using such a poor method to set its bearings.
JH

 

[john.k]

The thread seal cream type of anaerobic will lock those kinds of threads,and be movable if required.

 

[jim rozen]

There are some bearing nuts which are partly split, with a compression screw to lock the bearing in place. Once locked they will not move. Pratt Whitney used those a lot.

 

[Milland]

Any chance the spindle is slightly bent, or the thread on it is "drunken", or some other thing that could cause a cyclic tipping force on the nut face relative to the bearing race it abuts?

If none of those are a factor, clean the surface, retighten, and Loctite the poop out of that thing.

 

[deltap]

You can't go wrong with 2 lock nuts and a tab washer between. If they are coming loose you are not tightening them enough. Get them very tight and then tighten until tab aligns. One method is to make firm contact and then tighten another half turn or so depending on nut size and torque rating. Another very secure nut has 4 brass inserts threaded the same as the nut with Allen screws to force the inserts against the shaft threads.

 

[eKretz]

Tagged to watch that one^

Guessing that's pre-spam.

 

[Ksracer]

Thanks for the ideas guys. While a single Allen screw with a brass insert under it isn't the most secure method of preventing rotation, it seems like it should be adequate.

The nuts are backing away and the bearings are staying in place on the shaft. And both of them are doing it. I'd like to know what force could cause that. Certainly that set screw is able to overcome the inertia of accel and deceleration.

Securing the nuts with a more positive method is easy, but i'd like to know the root cause.

Thanks again

 

[Milland]

Tagged to watch that one^

Guessing that's pre-spam.

That name. That icon. A location of Bangladesh.

Seems legit to me...

 

[eKretz]

That name. That icon. A location of Bangladesh.

Seems legit to me...

Oh yeah. There IS a Delaware in Bangladesh, isn't there?

And I guess technically these days, that could be a Joanna?

 

[Milland]

Thanks for the ideas guys. While a single Allen screw with a brass insert under it isn't the most secure method of preventing rotation, it seems like it should be adequate.

The nuts are backing away and the bearings are staying in place on the shaft. And both of them are doing it. I'd like to know what force could cause that. Certainly that set screw is able to overcome the inertia of accel and deceleration.

Securing the nuts with a more positive method is easy, but i'd like to know the root cause.

Thanks again

Back both nuts off about three turns, then try tightening the set screw, then try moving the nuts. If the nuts still move easily, there's some failure of the retention screw. If they lock up, back the set screw out again, rotate further from the bearing, and examine the threads on the spindle at both locations where the set screw was bearing against and see if you notice any differences.

 

[Ksracer]

Back both nuts off about three turns, then try tightening the set screw, then try moving the nuts. If the nuts still move easily, there's some failure of the retention screw. If they lock up, back the set screw out again, rotate further from the bearing, and examine the threads on the spindle at both locations where the set screw was bearing against and see if you notice any differences.

I touched on that briefly in the first post. It takes a stiff tug on a 12" long spanner to move them with Allen screws tightened up. I haven't looked at the threads on the shaft, but that's a good idea.

I wonder if the brass has extruded sideways into the threads of the hole and isn't allowing to pressure to be transferred to the shaft?

 

[Richard King]

Midland and the others aren't as interested in why the brass lock doesn't work, they are telling you a better way to to repair it. The star lock washer will be a better repair. As long as there is a keyway cut into the spindle where the washer key can slide into it. Then try Loc-Tite blue or green on the nut threads. People here help find solutions and most could care less why the 2 nuts are turning.

 

[CarbideBob]

Thanks for the ideas guys. While a single Allen screw with a brass insert under it isn't the most secure method of preventing rotation, it seems like it should be adequate.

Actually this is the best method and way better than double nuts for precision bearings.
Is that brass piece free to move or has it gotten mushroomed on the top side?

 

[Gordon Heaton]

Actually this is the best method and way better than double nuts for precision bearings. . .

Yeah, for me getting a precise torque while trying to jam double nuts is very nearly impossible. I'm not very smart, though.